The xenotropic (X-tropic) mouse type C virus (MuLV), initially detected in the autoimmune NZB mouse, has been recovered from most mouse strains; it represents a common endogenous virus of house mice. This MuLV cannot exogenously infect mouse cells but can productively infect cells from many other mammalian and avian species. Cells from different mouse strains and even from one mouse differ in their spontaneous production of X-tropic MuLV. We have evidence that 2-3 different X-tropic MuLV are produced in one mouse tissue. We propose to isolate X-tropic MuLV subtypes from organs serologically and biochemically, and determine their differential expression in the mice and in individual mouse tissues. These and other related studies are aimed at elucidating the function of X-tropic MuLV in mice. Inherent in this research is the continued investigation on the antigen-specific non-immunoglobulin anti-X-tropic virus factor associated with mouse lipoproteins. We shall determine its size and structure and study the nature of its interaction with the virus. Once characterized and purified, its site of production in the mouse and its possible role in immune function and normal maturation processes will be evaluated. X-tropic viruses have been isolated from developing embryos, cells undergoing differentiation and normal and malignant adult tissues. We propose that the virus neutralizing factor influences certain stages of development by a specific interaction with X-tropic virus and/or its antigens on the cell surface. The study of this mouse model should provide insight into similar agents in other animal systems including man.